1. Field of the Invention
This invention relates generally to a system for converting wind energy into electric power, and pertains more particularly to a system in which a capacitively excited induction generator is driven by a wind turbine, the generator supplying a resistive heating load.
2. Description of the Prior Art
Conversion of wind energy into electric power is, of course, not new. Prior art systems can be broadly classified into three categories: (1) Constant-speed, constant-frequency system; (2) Variable-speed, constant-frequency system, and (3) Variable-speed, variable-frequency system.
The constant speed, constant-frequency system requires that the rotor speed be maintained constant by controlling the pitch angle of the turbine blades. This in turn will result in a constant electrical frequency if the blade adjustment is correct for the various wind speeds that are encountered. This arrangement may be necessary in installations employing very large blades where allowing the rotor speed to vary over a wide range might risk mechanical resonance. Unfortunately, the mechanical efficiency of the wind turbine, owing to the varying of the blade pitch, cannot be kept at its maximum value.
The variable-speed, constant-frequency system permits the turbine speed to vary with the wind speed, but at the same time requiring that a constant electrical frequency be maintained. In such arrangements, the generated alternating current is converted or rectified into direct current, and the direct current in turn inverted or reconverted to alternating current, the frequency usually being 60 Hz so that this type of system can be synchronized with and connected to existing power lines.
Other attempts have been made as far as producing systems having a constant frequency. However, they have all, as far as we know, required rectification and inversion in order to provide the requisite alternating current frequency.
The variable-speed, variable-frequency system is ideal where frequency insensitive heating loads are to be supplied with power. Since the frequency of the power furnished to resistive loads is not a factor to contend with, wind-generated alternating current, whatever its frequency due to the wind velocity at that particular moment, can be supplied directly to a heating load of this character. Studies, it might be pointed out, have indicated that in a residential home, a significant portion of the energy that is used is consumed for space heating and hot water heating. In such cases, wind energy is expected to find greater and greater economical application for supplementing the normal utility-supplied power (or the thermal power derived directly from fuel oil or propane gas, as the case may be) as fossil fuel costs continue to rise. Since the amount of wind energy is not constant at all times, reliance on fossil fuels cannot be entirely eliminated by utilizing wind power, but an appreciable reduction in their consumption can be achieved.